It is fundamental in the chemical industry that the temperature at which a particular reaction is carried out may be critical for successful optimum operation, and therefore it must be maintained within a specific range, which can be very narrow. Variations in the temperature outside this range can, for example, cause unwanted side reactions to occur, reducing the output of the wanted side product, and often requiring expensive further processing to remove the unwanted product or products. Chemical reactions are almost invariably either exothermic or endothermic, and reactions that do not involve heat generation or absorption are rare, so that it is usually essential to provide in combination with the reactor some means for adequate and precise heat exchange away from or into the reacting reactants in the reaction passage(s) or vessel.
Chemical and physical reactions cannot occur until molecules of the reagents are brought together one-on-one, and the required physical interaction is greatly facilitated as the reagents are more and more intimately mixed together. Bulk stirring is only able to the cause the reagent molecules to contact one another after sufficient time has elapsed to provide the necessary inter-dispersion. Typically the initial stirring shows bulk interaction taking place very quickly, for example within minutes, but interaction then slows considerably and satisfactory mixing may take several days to accomplish. One phenomenon that inhibits quick inter-dispersion is that the reagents in the bulk liquid inevitably are in the form of discrete bodies thereof, these bodies comprising what are known as Kolmogoroff smallest eddies or vortices, of dimension usually about 15-30 microns, which considerably reduce the possibility of the desired one-on-one molecular contact. Consequently, inside such Kolmogoroff eddies, only natural molecular diffusion can accomplish such one-on-one contact, which is a very slow process. The required encounters can be helped to occur by making the reactor of small enough scale that the dimensions of its reaction passage or passages are very small, ideally of the order of magnitude of the Kolmogoroff eddies, so that molecular diffusion now becomes much more significant. The role of such a reactor, and the mixing and mass transfer equipment associated with it, is to create sufficiently small scale fluid structures or eddies that the uniformity of mixing, mass transfer and molecular inter-diffusion is improved. Such reactors are generally referred to as micro-reactors, and many different types have been proposed. Micro reactors have the inherent property that the reactions involved take place within them at extremely high reaction rates, often requiring only milliseconds residence in the reaction passage. The provision of precise temperature control is exacerbated by the very small size and the special structures of such reactors.
There is described and claimed in my U.S. Pat. No. 7,780,927, issued 24 Aug. 2010, the disclosure of which is incorporated herein by this reference, what is now generally known as a spinning tube in tube reactor which, as its name implies, comprises a first cylindrical tube mounted within a second cylindrical tube of larger diameter, the tubes being relatively rotatable about a common longitudinal axis with the operative exterior surface of the inner tube spaced radially a very small distance (e.g. 300 micrometers or less) from the cooperating operative interior surface of the surrounding tube to provide an annular passage in which the reaction takes place. Usually the inner tube comprises a rotor, while the outer tube comprises a stator, although if required both tubes may be mounted for rotation. Tube in tube reactors with such small radial dimensions of the reaction passage come within the category of micro-reactors. The tubes usually are of uniform diameters along their lengths so that the reaction passage is of uniform radial spacing along its length, and through which the reactants pass while subjected to intense shear produced by their movement through the narrow passage, and by the relative rotation between the opposite tube surfaces. The reactor disclosed in this patent solves what has hitherto been a major problem with such reactors of ensuring that adequate uniformity is maintained in the radial spacing between the operative surfaces, despite the extremely small radial dimension. This is done by suspending the rotor within the stator solely by a flexible rotation-transmitting coupling and rotating the rotor at a speed such that the spacing is maintained by the hydrodynamic lubrication effect that occurs in the reactant liquid or liquid mixture passing upward in the reaction passage, which is of sufficiently small radial dimension for this phenomenon to occur.
In a reactor as described in the preceding paragraph the heat exchange structure will usually surround the stator so that the wall thereof is the heat exchanger member through which heat exchange takes place. It is of course possible to provide heat exchange means within the rotor with its wall as the corresponding heat exchange member, but this introduces practical difficulties of feeding the heat exchange fluid to the interior of the hollow rotating rotor without leakage, and removing it therefrom. Owing to the usual very small radial dimension of the reaction passage heat transfers very rapidly into or out of the passage while the temperature gradient in the radial direction remains sufficiently constant, so that normally it is sufficient to provide heat exchange means only with the stator. The size of the heat exchange structure is determined by the size of the reactor, or any other apparatus with which it is used, and therefore necessarily is small when the apparatus is small. The exchanger must therefore be as efficient as possible, and to this end it is preferred that the flow of heat exchange fluid through the heat exchanger passage or passages is laminar. Heat exchangers of the invention are able to provide such a flow which results in high heat exchange capability.
A phenomenon that can deleteriously affect the rate of heat transfer from a heat exchanger to its associated apparatus is that its flow passage or passages inherently have at the surfaces of their intervening walls a stationary layer of the heat exchange fluid through which the exchanging heat must pass. This is usually referred to as a boundary layer, and in the presence of a smooth flow will progressively increase in thickness up to a maximum value dependent on the fluid composition and its velocity. The boundary layer insulates the main body of the heat exchange fluid from the walls of the passage through which it is flowing and it is therefore of considerable advantage to keep this layer as thin as possible. One way of doing this is to interrupt the smooth fluid flow at appropriate intervals and in a way that the barrier layer thickness becomes as close to zero as possible; it will of course immediately begin to thicken again, so that the interruptions must be continued. The problem involved is to obtain successive continuous interruptions in the fluid flow without producing a flow that will increase the pumping pressure to an uneconomical value.
A constant problem with heat exchangers is the tendency for material from the heat exchange fluid to deposit on the walls of the heat exchange passage in which it is flowing. This material, however it originates, is generically referred to as “fouling material” and is deleterious to optimum operation of the heat exchanger by reducing the flow capacity of the heat exchange passage, and by reducing the rate of transfer of heat between the heat exchanger and the reactor through the intervening wall coated with the fouling material. The problem is especially difficult in heat exchangers employed with micro reactors owing to the necessarily small dimensions of the passages, so that the particles of fouling material can more easily block the fluid flow, and it is not unknown for the passages to require cleaning of the fouling material therefrom every few hours. The problem has been attacked from a number of different ways, such as using special heat exchange fluids, which may also be a highly purified, but these are usually expensive. Fouling can be substantially reduced, and even avoided, by moving the heat exchange fluid through the passage above a threshold velocity such that the fouling material can no longer adhere to the surface, but this usually requires such high pumping pressures as not to be economical, so that it is usually preferred instead to employ a lower pressure and accept that the exchanger must be cleaned at appropriate intervals.